Paraxylene (also “p-xylene” or “PX”) is generally considered the most important of C8 aromatic isomers, being used as an intermediate or starting material for such diverse end uses as synthetic fibers and bottle plastic. Paraxylene is typically obtained from a C8 aromatic hydrocarbon mixture derived from reformate by processes including aromatic extraction and fractional distillation. Although the composition of this starting C8 aromatic hydrocarbon mixture varies over a wide range, the mixture generally comprises 5 to 40 wt % ethylbenzene, with the balance, xylenes, being divided between approximately 50 wt % meta-xylene and 25 wt % each of para-xylene and ortho-xylene (this distribution considered the nominal “equilibrium concentration” of xylenes). Since, by some accounts, 80 wt % or more of the end use of xylenes involves the conversion of para-xylene to the above-mentioned end uses, obtaining para-xylene from its C8 isomers meta-xylene, ortho-xylene, and ethylbenzene, is the subject of a vast amount of continuing research.
By way of example, U.S. Pat. No. 5,004,855 teaches a process for the conversion of ethylbenzene in an aromatic hydrocarbon mixture comprising placing a C8 aromatic hydrocarbon mixture containing ethylbenzene and xylenes in the presence of hydrogen and in contact with a catalyst comprising rhenium, an acid type of a zeolite having a main cavity inlet composed of a 10-membered oxygen ring, and alumina, said catalyst having been subjected to a sulfiding treatment, to effect conversion of ethylbenzene to benzene. In embodiments, the ethylbenzene conversion step is conducted prior to and separately from the passage of the feed through the circulation system including para-xylene separation and xylene isomerization.
In U.S. Pat. No. 5,516,956 a mixture of aromatic hydrocarbons, comprising ethylbenzene and at least one xylene, is isomerized using a two component catalyst system to convert the ethylbenzene to compounds that may be removed from the aromatic hydrocarbon stream and to produce a product stream wherein the para-xylene concentration is approximately equal to the equilibrium concentration of the para-isomer. The first catalyst comprises an intermediate pore size zeolite that is effective for ethylbenzene conversion, and the second catalyst comprises an intermediate pore size zeolite, which further has a small crystal size and which is effective to catalyze xylene isomerization. Each of the catalysts of this invention may contain one or more hydrogenation components.
In U.S. Pat. No. 6,028,238, a process is described for isomerizing a feed which contains ethylbenzene and xylene, which process comprises the steps of: (a) contacting the feed under ethylbenzene conversion conditions with a particulate first catalyst component which comprises a molecular sieve having a constraint index of 1-12, the particles of said first catalyst component having a specified surface to volume ratio and the contacting step converting ethylbenzene in the feed to form an ethylbenzene-depleted product; and then (b) contacting the ethylbenzene-depleted product under xylene isomerization conditions with a second catalyst component.
Sulfur modification of a xylene isomerization catalyst is taught in U.S. Pat. No. 7,271,118. The catalyst comprises a Group VIII metal (referring to the traditional “CAS version” of the Periodic Table).
In prior art processes such as in the above-mentioned U.S. Pat. No. 6,028,238, a paraxylene-depleted C8 aromatics feed (meaning that the amount of paraxylene is less than the equilibrium concentration referred to above) is contacted with a catalyst system that de-alkylates ethylbenzene to benzene while isomerizing the xylenes to an equilibrium mixed xylene product. The ethylbenzene dealkylation and xylene isomerization reactions are advantageously accomplished in a dual-bed catalyst system. However, in commercial practice such units often experience large start-up exotherms during the initial oil-in period (contact of the catalyst with feed). An extreme bed temperature excursion can occur particularly when the liquid feed pump is incapable of delivering the hydrocarbon flow rate to a full design capacity within a short period of time. The resulting high hydrogen to hydrocarbon (H2/HC) molar ratio and high hydrogen partial pressure promote hydro-dealkylation and hydrogenolysis reactions, catalyzed by the hydrogenation metal component in the catalyst system, which in turn causes excess heat of reaction. Such a start-up exotherm can lead, among other negative consequences, to premature unit shutdown, mechanical failure of the equipment, poor isomerization performance, reduced catalyst life, and loss of xylenes, in commercial applications.
It is desirable to mitigate the start-up exotherm, so as to avoid negative consequences and maintain the high performance characteristics of the xylene isomerization catalyst.
The present inventors have surprisingly discovered a catalyst pre-treatment and start-up procedure that overcomes the disadvantages of the prior art system.